A representative of the Severodvinsk fire service however told AFP that "you can't smell the smoke in the city" and that municipal firefighters were not being called in to help the shipyard's own fire department.

The website of the Zvyozdochka (Star) shipyard said the Orel -- part of Russia's Northern fleet based in the region of Murmansk in the Barents Sea -- has been under repair since November 2013 and that the work would take two years.

Severodvinsk is located in Russia's northern Arkhangelsk region, on the White Sea.

In 2011, the rubberised coating on another nuclear sub called the Yekaterinburg caught fire while it was under repair in the northern port of Murmansk, injuring nine people who inhaled noxious fumes.